I've never heard of any disputed crew lists either. As important as it was to get it right for legal as well as financial reasons (You want to be sure that they pay envelope is going to somebody who actually exists!) the lines were pretty good about making it happen.
In any event, the identities of the 4th Engineers is also known and are Mr. Henry Watson Dodds, Mr. Henry Ryland Dyer, Mr. Leonard Hodgkinson, Mr. Thomas Kemp, and Mr. James Smith.
>>I've never seen White Star Line employ people that aren't British for the positions of engineers and such.<<
They may have but only the crew listings of their various ships could ever tell us for sure. Some may have been British subjects but from immigrant families. Regardless of that, they had to be excruciatingly careful about such things as crew listings and for much the same reasons that they had to be careful about passenger listings.
That doesn't rule out the possibility of an imposter but given that these people tended to know each other, and that engineers had to know what they were doing, I'm confident that a ringer would have been discovered very quickly.